Previously a predominant increase in the quantal content (m) with a smaller increase in quantal amplitude (v) has been revealed during the early (<1 h) phase of LTP (LTP1) in CA1 hippocampus. Our aim was to perform quantal analysis of the late (>2 h) phase of LTP (LTP2) with use of sharp microelectrode recordings. Only the cases with a high level of LTP (189-465%) were taken for the analysis (5 out of 91 inputs). The coefficient of variation of the response amplitude and the number of failures decreased, whereas in and to a lesser extend v increased after the tetanus. All changes were similar for LTP1 and LTP2, but the LTP magnitudes correlated differently with the initial m and the post-tetanic changes of paired-pulse facilitation. The results are compatible with the view of predominantly presynaptic LTP expression during both phases, but suggest mechanistic differences between the LTP1 and LTP2 expression mechanisms. The major suggested LTP1 mechanism is an increased probability of neurotransmitter release (Pr). LTP2 mechanisms could be related to mixed pre- and postsynaptic rearrangements, including: (i) increase of Pr; (ii) addition of new transmission zones; (iii) stronger synchronization of the neurotransmitter release; (iv) growth of new synapses. Independent of the underlying mechanisms, our data suggest only a small (if any) increase in the number of postsynaptic receptors per one transmission site.